1. Field of the Invention
The present invention relates to an oscillation element or oscillator configured to oscillate electromagnetic waves, and an imaging apparatus using the same.
2. Description of the Related Art
Heretofore, a configuration having an antenna (resonator) and a negative resistance element such as a resonant tunneling diode or the like is effective as an electromagnetic-wave oscillation element. There have been known an oscillation element or oscillator using a negative resistance element such as a resonant tunneling diode or the like and an antenna to generate electromagnetic waves including at least a part of frequency components of frequency bands from millimeter bands (30 GHz to 300 GHz) to terahertz bands (300 GHz to 30 THz).
Oscillation output of such an oscillation element tends to decrease along with increase in the oscillation frequency of electromagnetic waves to be oscillated. For example, though output as much as a mW class is obtained at around 100 GHz, output deteriorates to a μW class at around 1 THz. One cause for deterioration in oscillation output is miniaturization of a junction area of negative resistance elements along with increase in an oscillation frequency.
In general, it has been known that a negative resistance element such as a resonant tunneling diode or the like forms a RC low-pass filter function made up of a junction capacitor Cj and a serial resistor Rs equivalent to resistance from the negative resistance element to a resonator. Specifically, a high-frequency component of a signal is shielded by the RC low-pass filter function, and oscillating of electromagnetic waves in a higher frequency band than a cutoff frequency fc (=(2π×RsCj)−1) is not readily realized. Therefore, a cutoff frequency serving as the upper limit of a frequency band to be oscillated by an oscillation element has to be higher than the frequency band of electromagnetic waves to be oscillated. Here, the junction capacitor Cj is proportional to the junction area of a negative resistance element, and accordingly, the junction area has to be reduced as one method to increase the cutoff frequency fc (=(2π×RsCj)−1). On the other hand, the power of a power source which can be supplied to the negative resistance element is also proportional to the junction area of the negative resistance element. Accordingly, power which can be supplied to the negative resistance element is also decreased along with increase in an oscillation frequency, and oscillation output is also deteriorated.
Therefore, with U.S. Pat. No. 6,049,308 specification, in order to suppress deterioration in oscillation output, there has been disclosed a configuration of an oscillation element including multiple resonant tunneling diodes at a patch antenna resonator.
However, with U.S. Pat. No. 6,049,308 specification, multiple negative resistance elements are arrayed in a straight line, and accordingly, electromagnetic waves which all of the negative resistance elements oscillate have not been synchronized. Specifically, the multiple resistance elements are disposed with the common interval in the center portion of the antenna (resonator), and accordingly, there are negative resistance elements which mutually differ in the frequencies and phases of electromagnetic waves to be resonated and oscillated, and accordingly, the negative resistance elements have oscillated in a state in which electromagnetic waves having a different frequency and phase are mixed, which has in turn caused deterioration in oscillation output of the oscillation element.